2.3.1 Enthalpy Changes Exam Questions

Transcription

1 2.3.1 Enthalpy Changes Exam Questions 1. The standard enthalpy change of formation of hexane is 199 kj mol 1. Using the axes below, show the enthalpy profile diagram for the formation of hexane. On your diagram label the enthalpy change of reaction, H, and the activation energy, E a. enthalpy reaction pathway 2. Alkanes are important hydrocarbons since they are used as fuels in homes and in industry. It is important that the enthalpy changes involved in alkane reactions are known. [Total 3 marks] Define the term enthalpy change of formation of a compound. Write the equation, including state symbols, that accompanies the enthalpy change of formation of hexane, C 6 H 14 (l). (iii) What conditions of temperature and pressure are used when measuring the standard enthalpy change of formation? temperature... pressure... [1] [Total 5 marks] Greenhead College 1

2 3. The combustion of butane is shown in the equation below. C 4 H 10 (g) O 2 (g) 4CO 2 (g) 5H 2 O(l) The standard enthalpy change of combustion of butane is 2877 kj mol 1. What does standard mean in this context? [1] Define the term enthalpy change of combustion. (iii) Complete the enthalpy profile diagram for the combustion of butane. Label the activation energy, E a, and the enthalpy change, H. enthalpy C H (g) + 6 O (g) progress of reaction [3] [Total 6 marks] Greenhead College 2

3 4. In an experiment to determine the standard enthalpy change of combustion of propan- 1-ol, C 3 H 7 OH, a student used the apparatus shown below. thermometer beaker clamp water wick propan-1-ol (a) Define the term enthalpy change of combustion. (b) Write the equation for the standard enthalpy change of combustion of propan-1-ol, C 3 H 7 OH. (c) The student measured 50.0 cm 3 of water into the beaker and lit the burner. When the temperature of the water had gone up by 12.8 C, he found that g of propan-1-ol had been burnt. Calculate the energy, in kj, produced by burning g of propan-1-ol. The specific heat capacity of water is 4.18 J g 1 K 1. energy =... kj Calculate the number of moles of propan-1-ol in g. number of moles =... (iii) Calculate the enthalpy change of combustion, in kj mol 1, of propan-1-ol. enthalpy change... kj mol 1 [1] Greenhead College 3

4 (d) The student then calculated the error of the 50cm 3 measuring cylinder used. The maximum error of the measuring cylinder was +- 1cm 3. Calculate the % error in this piece of apparatus. [1] (e) The student looked in a text book and found that the actual value for the standard enthalpy change of combustion of propan-1-ol was more exothermic than the experimental value. Suggest two reasons for the difference between this value and the one he obtained experimentally (f) The student told one of her friends and she suggested using a 10cm 3 measuring cylinder 5 times as its maximum error was cm 3 State and explain whether this method would reduce the error in measuring 50cm 3 of water. [1] (g) The student repeated the experiment using the same apparatus. The water started at 60 o C. The student was surprised to find the value was less exothermic. Explain why. [1] (h) Other students in the class got very similar enthalpy changes of combustion even though their temperature changes and masses of propan 1 ol burnt were different. Explain this in terms of energy and moles. [1] [Total 15 marks] Greenhead College 4

5 5. A student carries out an experiment to determine the enthalpy change of combustion of glucose. In the experiment, g of glucose is burned. The energy released is used to heat 100 cm 3 of water from 23.7 C to 41.0 C. Calculate the energy released, in kj, during combustion of g glucose. The specific heat capacity of water = 4.18 J g 1 K 1. Density of water = 1.00 g cm 3. energy =... kj Calculate the amount, in moles, of glucose that is burned. amount =... mol (iii) Calculate the enthalpy change of combustion of glucose. Give your answer to three significant figures. ΔH C =... kj mol 1 [Total 6 marks] 6. Some reactions of H 2 O 2 are exothermic. Use ideas about the enthalpy changes that take place during bond breaking and bond making to explain why some reactions are exothermic [Total 2 marks] Greenhead College 5

6 7. The equations for the combination of gaseous atoms of carbon and hydrogen to form methane, CH 4, and ethane, C 2 H 6, are shown below. C(g) 4H(g) CH 4 (g) H = 1652 kj mol 1 2C(g) + 6H(g) C 2 H 6 (g) H = 2825 kj mol 1 Use this data to calculate: the bond enthalpy of a C H bond, bond enthalpy =... kj mol 1 [1] the bond enthalpy of a C C bond. bond enthalpy =... kj mol 1 8. Enthalpy changes can be calculated using enthalpy changes of combustion. The table below shows some values for standard enthalpy changes of combustion. [Total 3 marks] substance C(s) H 2 (g) CH 4 (g) H Ө c / kj mol Use these values to calculate the standard enthalpy change of the reaction below. C(s) 2H 2 (g) CH 4 (g) standard enthalpy change =... kj mol 1 [Total 3 marks] Greenhead College 6

7 9. Enthalpy changes of combustion can be used to determine enthalpy changes of formation. Write the equation for the standard enthalpy change of formation of butane, C 4 H 10. Include state symbols in your answer. Use the following data to calculate the standard enthalpy change of formation of butane. standard enthalpy change of combustion / kj mol 1 carbon 394 hydrogen 286 butane 2877 answer... kj mol 1 [3] [Total 5 marks] Greenhead College 7

8 10. The standard enthalpy change of combustion of glucose can also be determined indirectly. Calculate the standard enthalpy change of combustion of glucose using the standard enthalpy changes of formation below. substance O H f / kj mol 1 C 6 H 12 O 6 (s) 1250 CO 2 (g) 394 H 2 O(l) 286 C 6 H 12 O 6 (s) + 6O 2 (g) 6CO 2 (g) + 6H 2 O(l) answer =... kj mol 1 [Total 3 marks] Greenhead College 8